<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
  <style>
    .c0{
      margin: 0 auto; text-align:center;
    }
    .c1{
      display: block;
      margin-top: 20px;
    }
  </style>
</head>
<body>
<div class="c0">
  这是一个登录框
  <input class="c0 c1" type="text" placeholder="用户名">
  <input class="c0 c1" type="password" placeholder="密码">
  <input class="c0 c1" type="button" value="登录" onclick="clickme()">
</div>


<script type="text/javascript">
  function clickme() {
    alert('登录成功')
  }
</script>
</body>
</html>